Formation of neural circuits follows a stereotypical program across neural systems of axon extension, exuberant innervation and pruning of inputs that fail a selection process. Increasing evidence reveals this process to be an integrated effort by neurons and glia to build an orderly arrangement of cells and synaptic contacts. We have shown that formation of synaptic contact between the calyx of Held, perhaps the largest nerve terminal in the CNS, and its postsynaptic partner passes through a competitive phase during rapid growth over only a few days. The competitive phase has a sharp onset at postnatal day 2 in mice and rapid resolution of competition within 3 days for most MNTB cells, so the CH:MNTB system offers experimental advantages over other model systems. We will exploit novel technology, serial blockface scanning electron microscopy, across developmental age to provide a time-series view of all structural changes within this system that reveals all constituen cells at nanoscale resolution. Preliminary data suggest novel modes of physical interaction between growing calyces and glia in the developing brain and observed synaptic input onto glial cells at early neonatal ages. This unique data set, coupled to our novel whole-head slice preparation and genetic and viral vector tools developed during the last grant period, anchors investigations of the onset of neural activity from embryonic ages, roles for neural activity in calyx growth and competition, and potential synaptic communication among neurons and glia during neural circuit maturation. In sum, we will provide the first holistic view of neural circuit development that incorporates all cell types and describes circuit formation from the time of initial contact at embryonic ages to mature topography by hearing onset during the second postnatal week.